Apparatus for producing tremolo effects



March 7, 1961 c. L. FENDER APPARATUS FoR PRoDucING TR1-:Mom EFFECTS 2 Sheets-Sheet 1 Filed June 8, 1959 INVENTOR.

March 7, 1961 c. 1 FENDER APPARATUS FOR PRODUCING TREMOLO EFFECTS 2 Sheets-Sheet 2 Filed June 8, 1959 INVENTOR. L FE/VE@ CL4/@ENCE mun Arf-@MEX United States Pars-nf Ofi@ APPARATUS FOR PRODUCING TREMOLO EFFECTS i Clarence L. Fender, 2212 E. Revere, Fullerton, Calif. v Filed Jlllle 8, 1959, ser. N0. 818,752 I 12 Claims. (Cl. sli-1.25)

and method for producing a tremolo effect which is ex-v tremely pleasing to the ear and does not become oppressive or undesirable even at loud volumes.

Another object is to provide a tremolo means and method which do not produce undesired effects, such4 as altered pitch or volume, during rapid picking or note playing by -a guitarist or other musician.

A further object is to provide a tremolo means and method which is simple, reliable nad economical, and which operates properly at various tremolo frequencies and intensities.

A further object is to provide a tremolo circuit in which the tremolo action may be started or stopped at will, and in w-hichno undesired effects are produced when the tremolo is not employed.

Another object is to prov-ide a tremolo circuit adapted to be associated through a power amplifier with a loudspeaker, -and which does not prevent the feeding into said power amplilier of audio inputs from sources with relation to which no tremolo effect is desired.

A further object is to provide a tremolo circuit and method in which a tremolo effect is achieved throughout the entire audio range, including bass notes, without producing undesirable noises in the loudspeaker.

A further object is to provide a tremolo means and method adapted to cause a single loudspeaker to do a better job of handling both the higher and lower frequencies withoutdistortion or other undesirable effects.

A further object is to provide a tremolo circuit which,

produces a minimum of undesired effects such as distortion, chirping or beating in the loudspeaker.

A further object is to provide a tremolo circuit having the effect of causing a single loudspeaker to handle the higher frequencies cleanly `and without distortion due to doeppler or other effects, and which permits the ear of the listener to hear the higher frequencies and the lower frequencies alternately so that an intervening rest period occurs relative to each frequency band.

These and lother yobjects and advantages of theinvention will be set forth more fully in the following specification and claims, considered in connection with the attached drawings to which they relate.

In the drawings:

Figure 1 -is a schematic diagramv of an incorporating the present invention; and

electric circuit Figure 2 is a view which illustrates graphically and schematically the manner in which the present circuit and method produce a tremolo effect without varying the pitch of an entire note or the volume thereof.

It is pointed out that each note produced by a musical instrument or the like containsnot onlythe fundamental frequency but also many otherfrequencies-representing harmonics, distortion and the like. Throughout this Patented Mar. 7, 19631 specification and claims, each note will be referred to as having higher-frequency components, and lower-frequency components, as well as middle or intermediate-frequency components. These may also be referred to as higher, lower and middle-frequency bands, although such terminology is not entirely accurate since there is much overlapping.

Stated generally, the present invention relates to the concept of taking each note produced by a musical instrument or the human voice, and alternately increasing and decreasing the amplitudes of the higher-frequency components and the lower-frequency components in such note and in out-of-phase relationship. The out-of-phase relationship should be such that the higher-frequency components are at peak amplitude while the lower-frequency components are at minimum amplitude, and vice versa, there being a middle period or interval when both the higher-frequency and lower-frequency components, as

well as the intermediate-frequency components, come through strongly.

n The illustrated circuit generates land makes use of two equal and opposite tremolo waves, generally like those indicated at 10 and 11 in Figure,2 (curves 1 and 2). Such waves are at sub-audio tremolo frequency, normally in the range of about four to ten cycles per second, and are pre-ferably sinusoidal in shape. Audio frequencies from each note of the audio input are superimposed on the tremolo waves in such manner that the higher frequencies in each note are imposed on one wave (like wave 10) and the lower frequencies in each note are imposed on the other wave (like wave 11). For example, a higherfrequency wave is indicated schematically at 12 in curve 1 of Figure 2, and a lower-frequency wave is indicated at 13 in curve 2. Although the tremolo-frequency waves 10 and 11 are shown as being 180 out-ofphase with each other, it is pointed out that the audio-frequency waves 12 and 13 are in phase with each other. The amplitude of each superimposed audio wave 12 or 13 is then caused to vary cyclically in accordance with the tremolo wave but in 180 out-of-phase relationship relative to the other audio-frequency wave. Thus, the amplitude of wave 12 in the region 14 is illustrated as being at a maximumk trated graphically `at the -lower portion (curve 3) of Fig-- ure 2.

It is emphasized that the illustrated audio waves 12 and 13 are for purposes of explanation only and do not accurately lrepresent conventional audio waves, such audio waves being made up of many high, medium and low frequencies. The wave 12 is a schematic representation of one of the higher-frequency components and the wave 13 is a schematic representation of one of the lowerfrequency components, but man-y other high, low and middle-frequency components are involved. Furthermore, the shapes of waves 12 and 13 are misleading. The various middle-frequency waves are super-imposed on one or both of the tremolo waves 10 and 11, and thus appear in the output waves shownin curve 3 at the lower portion of Figure 2, but are not illustrated herein.

Referring to curve 3 of Figure 2, it is pointed out that the end product is neither a conventional tremolo nor a conventional vibrato, but is instead a tremolo in which predetermined bands of frequencies in each note are alternately sup-pressed and enhanced in a certain manner. The over-all pitch and amplitude or volume remain relatively constant, it being pointed out that at point 15 the sum of waves 12 and 13 is somewhat the same as the sum of such waves at points 14 and 16. Furthermore, it is emphasized that there are no nulls or dead spots, the amplitude at the intermediate point 15 being substantial with respect to both higher and lower frequencies as well as the unshown intermediate frequencies.

lt is a feature of the invention that the various middle or intermediate-frequency components' of each'note are transmitted to the loudspeaker at relatively uniform strength, being much less affected by the tremolo means than `are the higher and lower-frequency components.

Thus, a particular middle frequency may be superimposedl on both of the tremolo-frequency waves (like'waves 10 and 11), so that upon recombination (as in curve 3'of` l 'balancing phase inverter 20 is provided in order to produce at circuit point 21 a sinusoidal voltage wave (generally like wave 11) which is the substantial mirror image of the wave at point 19. Circuit means 22 are provided to impress upon the waves present at points 19 and -21- audio frequencies which are respectively in the upperfrequency and lower-frequency ranges for each note, and to effect amplitude variation in such upper and lowerf Vfrequency ranges in out-of-phase relationship relative to each other. More specifically, and as will be described in detail subsequently, the circuit means 22 incorporates components adapted to separate the audio input in'to a treble band and a' wide-range band, the latter band be# coming mainlyV a bass band due to negative feedback of general type described in my previous Patent No.7 2,817,708, issuedrDecember 24, 1957, for an AmplifierV With Tremolo.` VThus,-`capa`citors -26 and -27 of the' present circuit correspond to capacitors 89 and' 92 of the above-mentioned patent, and resistor 28 of the-present circuit corresponds to resistor 90 of the patent. l The' frequency of the oscillations vin the circuit, which a phase-shift oscillator, 'are controlled by varying the setting of a variable resistor 29. Oscillations may be stopped and rapidly re-initiated by closing and opening -a switch 30 which provides a short to ground from the circuit point between capacitors 26 and 27. I As described in the above-cited patent, the phase-shift oscillator incorporates a triode 31 from the'plate` of which the oscillator output is derived, being impressed across a plate resistor 32. Such oscillator output is fed through a suitable filtering and amplitude-control means including a capacitor 33 and three resistors 34-36 connected in series with each other between the plate of triode 31 and the previously-mentioned circuit point 19. A capacitor 37 is connected between the junction ofA capacitors 34 and 35 and ground, and a variable resistor 38 is connected between the junction of resistors 35 and 36 and ground.

The capacitor 33 is a blocking capacitor for blocking the direct voltage impressed upon the plate of triode 31 through resistor 32 from a suitable plate voltage source. Resistor 34 and capacitor 37 combine to form a lowpass filter adapted to smooth the wave from the phase-- shift oscillator and cause it to be essentially sinusoidalA as previously stated. Resistors 3 5 and 38 form avoltage' 4 divider and a shunt attenuator 4adapted to limit and control the amplitude of the signal delivered to circuit point 19 from the phase-shift oscillator 18. Resistor 36 is an isolating resistor which prevents resistor 38 from loading the audio input signal and also one of the tubes in the circuit means 22 to be described hereinafter.

It is to be understood that the circuit 13 may comprise any suitable means for delivering a relatively 4pure sinusoidal' wave to circuit point 19 at a subaudio tremolo frequency such as between 4 and l0 c.p.s. Thus, it is not believed to be necessary to present a further description of the various components in the phase-shift oscillator, etc., indicated aboveand in the cited patent. Additional components of the.' illustrated oscillatorv are numbered 29a, 30a, 30b, 30e and 30d.

Proceeding neXt to a description of the self-balancing phase inverter 20, this comprises two equal resistors 40 and 41'connected in series with each other betweenthe circuit points 19 and 21. Circuit point 21 is connected through a series-connected resistor 42 and capacitor 43 to the plate of a second triode 44. The triode 44 is preferably a high-gain amplifying tube, having a plate resistor 46 connected to the previously-mentioned plate resistor 32 for triode 31. B+ voltage is supplied to the plates of both triodes through their resistors 32 and 46 fromy The triode 44 has its cathode connected through 'a suitable cathode resistorI 53 to ground. The previously-A indicated capacitor 43 and resistor 42 in the plate circuit of triode 44 perform blocking and isolating functions to bl'oclc the B-lvoltage on triode `44 and to prevent loadingv of one of the triodes in circuit means 22.

-It v-is an important feature of the self-balancing phase inverter 20 that the grid of triode 44 is connected through a lead 56vto the junction point 56a between equal resistors 40 and 41.- Furthermore, such junction point 56a isV connected through a parallel-connected resistor 57 and capacitor 58 to ground. The capacitor S8 has a value .j which' is sufficiently-high that all audio frequencies present a't point 56a will be by-passed to ground, but suicientlylow toprevent by-pass of the tremolo frequency to ground. Resistor 57 is sufficiently high to prevent substantial shorting of 'current from the 'above-mentioned junction point to ground.

point 19 or circuit point 21 to junction 56a is by-passed In the operation of the self-balancing phase inverter 20, thepure Asinusoidal wave present at circuit point 19 is transmitted through resistor 40 and lead 56 to the control grid of triode 44. Any audio transmitted from circuit l a throughcapacitor 58 to ground and thus has no effect on the triode 44. Furthermore, such by-passing or shunting of audio to ground through capacitor 58 maintains separate the signals impressed upon the grids of the double triode which forms part of the circuit means 22 to be' described hereinafter. l Y

The sinusoidal tremolo-frequency wave impressed upon the grid of triode 44, as described above, produces au inverse or negative lfeedback eiect that causes the triode 44 to attempt to mak'e the voltages at points 19 and 21y ground as previously stated, it follows that the triode 44 I has the effect of'duplicating a pure sinusoidal wave at:

circuit point 21 but in inverse-phase relationship.

l roceeding next to a description of the circuit means 22:

for .impressing the higher-frequency, and lower-frequency components of each audio note on the tremolo-frequency waves respectively presentat points 1-9 and 21, and for effecting cyclic variations in the amplitudes of such audio components :inl accordancewith the tremolo waves, such means includes the previously-indicated double-triode tube comprising two triodes 61 and 62.l Leads 63 and 64 are employed to effect,U respectively, direct connections between circuit points 19 and 21 and the grids of the triodes 61 and 62. The cathodes of the triodes are connected to each other through a lead 66, and lead 66 is lconnected to ground through the parallel-connected resistor 67 and capacitor 68 which perform important functions lto beset forthhereinafter. Connected between the plates of thepvtrfiodes a(S1-"and, 6'2 are series-connected plate resistors 69 and 70 having equal values. Plate voltage is thus supplied `to the plates from the previouslyindicated B+ source 47, by means of a lead 71 which connects not only to lead 48 but to the junction between resistors 69 and 70.

Capacitors 72 and 73 are respectively connected to the leads 63 and 64 and are also connected to a resistor 74 in such relation that the elements 72-74 are in series across the leads 63 and 64. A suitable audio input source, indicated` at 75, is connected through a lead 76 to the junction between capacitor 72 and resistor 74. Such fsource75 may comprise, for example, the electromagnetic pickup of an electric guitar, a microphone, etc., and may include preamplifier means.

In the operation of the circuit means 22 as thus far described, let it be assumed that a single audio note is fed from source 75 and through lead 76 to the junction of capacitor 72 andresistor 74. This note may comprise, to give just one of very many examples, middle C as .produced by an electric guitar. It is to be understood that the operation is the same when many notes are fed inltat; the` Sametime, -as when a chord is struck or when several instruments (or phonographs or microphones) are connected to the input.

Tlf-he capacitor 7,2 is selected to have a relatively low value which will block the lower andmiddle frequencies of such note but will permit passage of the higher frequencies thereof to lead 63 land thus to the grid of triode 61. Such higher frequencies are thus superimposed 'onthe'sine wave (generally like wave 10 of Figure 2) which is'delivered directly from circuit point 19 to the .gridof triode 61. Triode 61 is selected to have such a non-linear characteristic that the amount of amplificationfof Asuch higher-frequency audio components will be vgreater when the tremolo sine wave is in the positive .portion of its cycle, as `indicated at region 14 in Figure 2, than when i-t is in the negative portion of its cycle as ndicatedat .region 16 therein. The resultA is an output wave. schematically represented by curve 1 of Figure 2. Capacitor 72 also performs the function of preventing the tremolo-frequency wave from affecting the audio input.

p A relatively wide band of frequencies, derived from thegnote fed in from source 7S through lead 76 as stated above, is .passed by the resistor 74 and medium-value -capacitor 73. to lead 64. Such relatively wide band is,

however, converted into a bass band by means of a low value capacitor 77 which is interposed in a lead 78 between the plate-and grid of triode 62. Since capacitor 77 has a low value, it'effects inverse or negative feedback only of the higher frequencies and thus has the effect of attenuating the higher frequencies which are passed tothe .plate resistor 70 of triode 62. The result is that the voltage present at the output of triode 62 is in the 'nature of the middlewave (curve 2) shown in Figure 2, incorporating lower frequencies impressed upon the Vsinusoidal tremolo-frequency 'wave which was fed directly Atotliegridlof triode 62 from circuit point 2li. as previously stated.

` ilitliecase of'th'eftriode 61, triode 62 has the nonlinear effect of performing greater amplification of the audio when the tremolo-frequency wave is in its positive half-cycle than when it is in its negative half-cycle. However, and as shown inthe upper two curves in Figure 42, the out-of-phase relationship between waves 10 and 11 causes the amplitude variations between the higherand lower-audio frequencies to be 180 outof-phase. It is to be remembered, however, that, although the amplitudes vary in out-o'f-phase relationship, the higher and lower-frequency components of the note are actually in phase with each other. By `this it is meant that no audio signal is cancelled out when 4the higher-frequency and lower-frequency components are combined. H l Y y The capacitor 73 is selected in such manner that it is large enough to pass the lower audio frequencies from lead '76.to lead 64,V but small enough toprevent the tremolo-frequency voltage to pass from lead 64 to lead 76 and affect the audio input. Resistor 74 performs the function of aiding in reducingk the higher frequencies which, as stated above, are effectively attenuated by negative feedback through capacitor 77.

It is emphasized that the tremolo-frequency waves are isolated from the audio input by capacitors 72 and 73, as stated above, and that there is no substantial interaction between the audio components in leads 63 and 64. Thus, as previously stated, the audio frequencies which attempt to pass between leads 63 and 64 through seriesconnected resistors 40 and k41 are shunted to ground through capacitor 58. Furthermore, should any audio frequencies penetrate to the point between the plate resistors 32 and 46, they are shunted to ground through capacitor 52.

There will next be described -the very important functions of the cathode resistor 67 and capacitor 68. Resistor 67 is caused to have a relatively high value in relation to normal amplifier practice for the particular plate resistor 69 and triode 61. On the other hand, the resistor'67 should have a lower value than in normal circuits in which the triode is used as a phase splitter, one reason for this being because it is desired that triode 61 perform amplification. Capacitor 68 has a value suf# iiciently high to pass substantially all `of the audio frequencies but sufficiently low to prevent passage of the trcmolo frequencies. Thus, the audio frequencies are shunted to ground through the capacitor 68, whereas the tremolo -frequencies must pass through the cathode resistor 67. It follows that in discussing the function of resistor 67, the tremolo frequencies are all that are referred to.

The high-value cathode resistor 67 has been found to be extremely important in creating greater trernolo (differential amplification) action in the triodesy 61 and 62, so that the tremolo-frequency waves need only have relatively small amplitudes. This may best be understood by considering the situation when the grid of triode 61 is positive and the grid of triode 62 is negative. When the grid of triode 61 is thus relatively positive, the ow of plate current therein is increased. This causes a voltage change across resistor 67 which is such that the end `of such resistor adjacent lead 66 is more positive than previously, causing the cathode of triode 62 to be more positive than previously. Such increase in the positive condition of the cathode of triode `62 has the effect of making the grid of such triode more negative with respect to the cathode thereof, so that the flow of plate current in triode 62 is much less than would be the situation if resistor 67 had only a lower (normal) value with consequent small voltage thereacross. This reduction in the plate current in triode 62 (change in the operating point) reduces the amount of amplification of the superimposed audio components, as is desired. lt is to be understood that the reverse action occurs when the grid of triode 61 is negative and that of triode 62 is positive, the ow of plate current in triode 61 7 then being much lessvthan would be the case if cathode resistor 67 were of normal (smaller) size.

Although.v reference was made in the preceding paragraph to the grids of triode 61 and 62 becoming positive and negative, it is to be understood that these are only relative terms and that the grid of neither tube should actually become sufficiently positive to effect grid-current clipping or distortion ofthe audio components. The cathode resistor 67 plays an important role in this also, since such high-value resistor causes the cathode voltages to uctuate with the grid voltages in such manner that grid-current clipping or distortion will not occur unless the amplitude of the tremolo-frequency wave is great. Stated otherwise, the voltage on `the cathode of each tube iluctuates along with the voltage on the grid of such tube, to a certain degree, and in such manner that the grid may become more positive without resulting in undesirable grid current and thus in clipping and distortion ofthe audio. Although the cathode voltage and grid voltage of each tube thus fluctuate somewhat together, the cathode voltage on each tube uctuates oppositely eraser audio input therefore passes through lead 76 and bothv of the triodes 61 and 62 to output 84 in a substantially unaffected manner as is desired. f

When tremolo action is desired, the operator opens the' switch 30 to immediately initiate oscillations in the phaseshift oscillator circuit 18 and in the manner described in the cited patent.` The resulting tremolo-frequency wave, having a frequency determined by the setting of resistor 29 and an amplitude determined by the setting of resistor 38, is therefore transmitted to the circuit point 19. Such wave may correspond generally to the wave 10 indicated by curve 1 of Figure 2, as stated previously. A substantial mirror image of the tremolo wave present at point 19 is created by the self-balancing phase inverter 20 and caused to be present at the circuit point 21, this wave being generally like the one represented by wave 1 1 (curve 2 of Figure 2). This action A is substantially unaffected by the audio-frequency comto the grid voltage on the opposite tube to create the connected through a coupling capacitor 83 to the audio power amplifier and loudspeaker 84. It is a feature of the present invention that other inputs may also be connected to such power'amplifier, without passing through the present tremolo circuit, in instances where no tremolo is desired.

Since the resistors 81 and 82 are equal, and since the tremolo-frequency waves impressed thereacross are of equal magnitude and are 180 out-of-phase with each other, 'it follows that such waves will be eliminated and will not be present in the audio output 84. It is an important feature of the invention that the tremolofrequency waves have equal magnitudes, and thus cancel properly, `despite variations n the tremolo frequency due to adjustment of resistor 29. A change in tremolo frequency has the strong tendency to cause waves 10 and 11 to become unbalanced, since the capacitors 72, 73 and 77 act as loads which become greatly unbalanced when the tremolo frequency is varied. Thus, the selfbalancing operation of circuit 20` is extremely important in maintaining the tremolo waves in balance so that there will be perfect cancellation and no pulsing in the loudspeaker.

As previously stated, the audio frequency components are in phase with each other and are therefore combined and passed to the audio output 84 in a wave having the general nature of the one shown in curve 3 at the bottom of Figure 2. This desirable action would not occur, however, were it not for the cathode capacitor 68 which prevents triodes 61-62 from acting as phase inverters. Stated otherwise, capacitor 68 prevents the audio signal on one triode 61-62 from producing an inverted signal at the other, with consequent cancellation at mixer 81- 82. In summary, therefore, the second double triode 61-62 is caused to lact eiciently as an amplifier at the audio frequencies, having a cathode resistor 67 and capacitor 68. At tremolo frequencies, tube 61-62 operates as a tube with no cathode capacitor, and resistor 67 provides the important biasing effect stated heretofore.

METHOD A D vSUMMARY oF OPERATION y Let it .be assumed that-one or more musical instruments, microphones, .phonographs, etc., are connected to the audio input 75. During periods when no tremolo action "is desired, the switch 30 is maintained in closed condition to prevent oscillations in the circuit 18. The

ponents due to the presence of the capacitor 58 which shunts the audio frequencies to ground.

The incoming audio is separated into a treble band by the capacitor 72, and a wide-range band by resistor 74 and capacitor 73. The treble band is impressed upon the tremolo-frequency wave in lead 63, and by differential action of tube 61 is amplified more greatly when such tube is relatively positive than when it is relatively negative, the result being represented by the wave 12 indicated in curve 1 of Figure 2. l

The wide-range band passed to lead 64 by resistor 74 and capacitor 73 is impressed upon the tremolo-frequency wave in lead 64. By differential action of triode 62, and because of negative feedback of the higher-frequency components through capacitor 77, the resulting wave at the output of triode 62 is in the general nature of wave 13 shown in curve 2 of Figure 2. As previously stated in detail, the cathode resistor 67 and capacitor 68 perform extremely important functions in the proper action of the triodes 61 and 62. l

The output waves from triodes-61 and 62 are combined?, by means of resistors 81 and 482, to form the wave -generally represented at the bottom (curve 3) of Figure 2, the tremolo frequencies having been cancelled and the audio frequencies having been added.

The result is the creation of a tremolo audio output which is extremely pleasing to the ear, even at loud volumes, and produces great advantages not only in connection with loudspeakers for electrical musical instruments or vocalists but in connection with radios, phongraphs, and the like. To name only some of these ad'- vantages,l all of the bass frequencies are passed through the tremolo circuit yet there is no rumbling noise produced in the loudspeaker. The loudspeaker is permitted to do a much better job of handling the high frequencies, without distortion due to the doeppler effect and other factors, because the bass frequencies are relatively weak when the high frequencies are relatively strong. Furthermore, since the ear of the listener is not required to listen to bass and treble frequencies at the same time and of equal magnitude, the listener is conscious ofa more pleasant and desirable sound than heretofore. This effect is in some respects comparable to persistence of vision.

There is no substantial variation in over-al1 volume, or in the over-all pitch of each note, with the result that sharp and rapid picking by a guitarist (or comparable action in another instrument) does not appear to produce relatively high or low notes, or weak notes, in irregular and unpredictable array. This is to be contrasted with a conventional amplitude tremolo, for example, in which a sharp pick performed during a low-amplitude portion of the cycle would appear undesirably soft with relation to the other notes. It is a very pleasing and important feature of the present tremolo effect that the tremolo does not appear to. become evident except. onrelativel'y susl tained notes'.

9 SPECIFIC EXAMPLE The following vis 'fa specific example of circuit values which may be employed in the circuit of Figure 1, allcapacitance values being given in vmicrofarads and resistance values being givenihn kil'ohms. Each of the double triodes may be a lZAXf-. The B+ voltage at 47 may be 350vo'1ts;

It is to be understood that, throughout this specification 'and claims, reference to the triode vacuum tubes (and components thereof) applies also to comparable vacuum tubes or transistors or like electronic currentflow control devices. Thus, the cathode of each vacuum tube would normally correspond to the emissive element or emitter of a transistor, the plate of each vacuum tube would normally correspond to the collecting electrode or collector of the transistor, and the grid of each vacuum tube would normally correspond to the base of the transistor.

Various embodiments of the present invention, in addition to what has been illustrated and described in detail, may be employed without departing from the scope of the accompanying claims.

I claim:

l. Apparatus for producing an effect in the nature of a tremolo relative to a musical source adapted to generate audio-frequency musical waves which are complex and rich in harmonics, which apparatus comprises first electric circuit means for selecting from a complex wave generated by said source a first band containing predominantly the higher-order harmonics present in said complex Wave, second electric circuit means for selecting from the same complex wave generated by said source a second band overlapping said first band and containing predominantly the lower-order harmonics present in said complex wave, tremolo means associated with said first electric circuit means to amplitude-modulate the signals present in said first band at a predetermined tremolo frequency, tremolo means associated with said second electric circuit means to amplitude-modulate the signals present in said second band at the same predetermined tremolo frequency but in substantially out-of-phase relationship relative to the amplitude modulation of the signals present in said first band, and means associated with said tremolo means to transmit the amplitudemodulated signals in said first and second bands to loudspeaker means, whereby the listener to said loudspeaker means is alternately conscious of the signals in said first band and then in said second hand at said predetermined tremolo frequency.

2. The invention as claimed in claim l, in which said tremolo means include means to effect amplitude-module# tion of the signals present in saidlirst band in substantially `180 degree out-of-phase `relationship relative to lthe amplitude-modulation of the signals present in said second band, and in which said predetermined tremolo frequency is in the range of four to ten cycles per second. l

3. The invention as claimed in claim 1, in which said tremolo means include means to effect generally sinusoidal amplitude-modulation of the signals present in said first band, and generally sinusoidal amplitudemodulation of the signals present in said second band in substantially 180 degree out-of-phase relationship relative to the amplitude-modulation' of the signals presentv in said first band, said tremolo means being such that the amplitude modulations lare of substantially equal degree whereby the signals present in said first band and in said second band have substantially the same magnitude at intervals degrees out-of-phase with the peak amplitudes of the signals present in either band.

4. The invention as claimed in claim 1, in which said apparatus includes means for generating and conducting to` a single circuit point electricalsignals corresponding to said complex wave, and in which said first and second electric circuit means include segregation means connected to said circuit point to at least partially segregate the electrical signals present thereat into said first and second overlapping bands.

5. The invention as claimed in claim 1, in which said electric circuit means and tremolo means are constructed to cause the signals present in said first and second bands to remain in phase with each other.

6. The invention las claimed in claim 1, in which said tremolo means include means for generating at least two generally sinusoidal tremolo-frequency voltage waves having the same frequency, means for superimposing on one of said voltage waves the signals present in said first band, means for superimposing on the other of said voltage waves the signals present in said second band, means for causing the amplitudes of said superimposed signals on each of said voltage waves to vary at the frequency of such voltage waves, means for causing said amplitude variations to be out of phase relative to each other, and means for eliminating said tremolo-frequency voltage waves,

7. The invention as claimed in claim 1, in which said tremolo means include means to generate a first tremolofrequency voltage wave in a sub-audio range, means to generate a second tremolo-frequency voltage wave which is a substantial duplicate of said first wave and is in phase inversion relative thereto, means to superimpose the signals present in said first band onto said first voltage wave and the signals present in said second band onto said second voltage wave, said last-named means being adapted to cause said signals present in said first band and in said second band to be in phase with each other, electronic current-control means to cause the amplitudes of said superimposed signals to vary cyclically in out-ofphase relation relative to each other and in accordance with said first `and second voltage waves, and means to cancel said first and second voltage waves.

8. The invention as claimed in claim 7, in which said means to generate a second voltage wave which is a substantial duplicate of said first wave comprises a selfbalancing phase inverter circuit.

9. The invention as claimed in cl-aim 7, in which said electronic current-control means comprises first and second vacuum tubes having non-linear amplification characteristics, in which said vacuum tubes each have a plate, cathode and control grid, in which capacitor means are provided between the cathodes of both of said vacuum tubes and ground, said capacitor means having a capacitance sufficiently low to prevent passage of sub-audio tremolo-frequency currents to ground and sutiiciently high to permit passage of audio-frequency currents to ground, in which means are provided to impress respectively on maaar relatively high-value cathode resistor means are provided in parallel relation to said capacitor means, said cathode resistor means being suciently high in value that when the control grid of one of said vacuum tubes is in a relatively positive condition with respect lto its cathode the control grid of the other of said vacuum tubes will be in a more negative condition with respect to its cathode than would be the case in the absence of said relatively high-value cathode resistor means.

10. The invention as claimed in claim 9, in which said cathode resistor means is much higher in value than conventional circuit practice with respect to the particular vacuum tubes and lload when the vacuum tubes are employed as ampliers, and is lower in value than conventional circuit practice for the particular vacuum tubes and load when the vacuum tubes are employed as phase splitters.

11. The invention as claimed in claim 1, in which said first electric circuit means comprises electromagnetic 1'2 pickup means ofan electric guitar and filter means to select predominantly the higher-order harmonics present in the complex'wave generated by the guitar and sensed by said pickupmeans, and in which said second electric circuit means comprises said electromagnetic pickup means of the, guitar and second ltermeans forselectng from the same'complex wave generated by said guitar and sensed by said pickup means predominantly the lower-order harmonics present in said complex wave.

12. The invention as claimed in claim 1l, in which said pickup means comprises a single electromagnetic pickup.

,References Cited in the tile of this patent UNITED STATES PATENTS 2,583,566 Hanert Ian. 29, 1952 2,835,814 Dorf May 20, 1958 2,845,598 Meyer July 29, 1958 2,892,372 Bauer June 30, 1959 2,892,373 Bauer June 30, 1959 2,899,644 vLeslie Aug. 11, 1959 

